Single crystal silicon, which is the starting material for most processes for the fabrication of semiconductor electronic components, is commonly prepared with the so-called Czochralski (Cz) process wherein a single seed crystal is immersed into molten silicon and then grown by slow extraction. Molten silicon is contaminated with various impurities, among which is mainly oxygen, during the time it is contained in a quartz crucible. At the temperature of the silicon molten mass, oxygen comes into the crystal lattice until it reaches a concentration determined by the solubility of oxygen in silicon at the temperature of the molten mass and by the actual segregation coefficient of oxygen in the solidified silicon. Such concentrations are greater than the solubility of oxygen in solid silicon at temperatures typical for the processes used to fabricate electronic devices. As the crystal grows from the molten mass and cools, therefore, the solubility of oxygen in it decreases rapidly, whereby in the wafers sliced from the crystal, oxygen is present in supersaturated concentrations.
Thermal treatment cycles typically employed in the fabrication of electronic devices can cause the precipitation of oxygen in silicon wafers which are supersaturated in oxygen. Depending upon their location in the wafer, the precipitates can be harmful or beneficial. Oxygen precipitates located in the active device region of the wafer can impair the operation of the device. Oxygen precipitates located in the bulk of the wafer, however, are capable of trapping undesired metal impurities that may come into contact with the wafer. The use of oxygen precipitates located in the bulk of the wafer to trap metals is commonly referred to as internal or intrinsic gettering (“IG”).
Some applications, such as power devices including MOSFETS, require heavily arsenic or phosphorous doped, low resistivity single crystal silicon wafers having intrinsic gettering. Such wafers have presented challenges. At the concentrations of arsenic and phosphorous required to achieve the desired resistivity of 5 mΩ-cm or less, the arsenic or phosphorous dopant tends to suppress oxygen precipitation below the threshold density desired for intrinsic gettering.
In U.S. Pat. No. 6,491,752, Kirscht et al. attempted to address this problem by doping the single crystal silicon with carbon during the crystal growth process. The amount of carbon varied from relatively light doping (˜4×1016 atoms/cm3) near the seed end of the single crystal silicon ingot to relatively heavy doping (˜2×1017 atoms/cm3) near the tail end of the single crystal silicon ingot; according to Kirscht et al., heavy carbon doping was required to yield wafers in which oxygen precipitates could form if the wafer was sliced from a location of any appreciable distance from the ingot seed cone. This approach, however, was not entirely satisfactory. For example, oxygen dependent intrinsic gettering was not observed for heavily doped wafers sliced from about the mid-point along the crystal axis to the end cone. See Kirscht et al.'s FIG. 4. Also, this approach requires carbon concentration to increase as a function of ingot length; resulting in a lack of wafer-to-wafer uniformity and more complex crystal growth process control.